This invention relates to superalloy articles such as used in aircraft gas turbine engines, and, more particularly, to such an article having an abradable, thermally densified coating applied thereto.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of hot exhaust gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gas flows from the back of the engine, driving it and the aircraft forwardly.
The turbine includes a turbine disk and a large number of turbine blades extending radially therefrom. The hot exhaust gas is directed against the turbine blades. To prevent leakage of hot gas past the tips of the turbine blades, the turbine has a stationary shroud that acts in the manner of a tunnel at the perimeter of the turbine. A seal is provided on the inner surface of the shroud to minimize hot gas leakage. It is common practice to form at least the sealing portion of the shroud from an abradable alloy. The clearance between the turbine blade tips and the seal is made to a tightly controlled tolerance. The clearance is designed to be minimal at steady state operating conditions. During takeoff and thrust reverser conditions, some incursion of the blade against the shroud may occur, with the shroud being a sacrificial component that is worn away as necessary by the rubbing of the blade tips.
After the gas turbine engine has been in service, there is a gradual wearing away of the tips of the turbine blades by hot gas erosion. As this wear continues, the clearance with the shroud, and the amount of hot exhaust gas leakage, gradually increases. The result is decreased engine efficiency. It is therefore common practice to refurbish the shroud by adding a coating of an alloy to the inner surface of the shroud. The sealing described above is again achieved, and efficiency is restored.
The hotter the exhaust gas, the more efficient is the operation of the gas turbine. The maximum operating temperatures of gas turbines has therefore gradually increased over the years as a result of improvements in engine design and the materials of construction. One area of improvement has been to use shrouds made of nickel-base superalloys rather than cobalt-base superalloys. The nickel-base superalloy shrouds are desirably made of materials with directionally oriented microstructures achieved by directional solidification, which can be used at higher temperatures than conventional non-directional microstructures.
Materials for use on shrouds having non-directional microstructures are well known. It has been found that these materials do not function well for refurbishment of shrouds made of nickel-base superalloys having directional microstructures, which operate at temperatures above the capability of the available materials. The current cobalt-base materials used in shroud refurbishment are incompatible with the directionally oriented nickel-base shrouds for the temperatures at which the shroud is operating.
There is a need for seal materials operable with nickel-base superalloys having directional microstructures, either oriented polycrystals or single crystals. The present invention fulfills this need, and further provides related advantages.